HK1150680B - Tunable acoustic reflector - Google Patents

Description

Tunable acoustic reflector

Technical Field

The present invention relates to an acoustic reflector, and in particular to an underwater reflection target used as a navigation device for positioning and repositioning.

Background

Underwater reflective targets are typically acoustic reflectors that are generally used in sonar systems (e.g., for marking underwater structures). Relocating devices are used, for example, to identify pipes, cables and minerals, and also in the fishing industry to acoustically mark nets.

To be effective, the acoustic reflector needs to be easily distinguished from background features and environmental clutter, so the reflective target is expected to: (a) a strong reflected acoustic output response (i.e., high target intensity) can be generated relative to the intensity of the acoustic wave reflecting the background features and ambient clutter; (b) with acoustic properties that can distinguish it from other (false) targets.

Enhanced reflection of acoustic waves from a target is currently achieved by refracting the incoming acoustic waves incident on one side of a spherical shell so that they are focused along an input path on the opposite side of the shell from which they are then reflected and emitted as an output reflected response by a reflector. Alternatively, the input sound wave may be reflected multiple times from the opposite side of the housing of the reflector before being emitted as an output reflected wave.

Known underwater reflective targets include a fluid-filled spherical shell. The fluid-filled spherical shell target has a high target strength when the acoustic velocity of the selected fluid is about 840 m/s. Currently, this is achieved by using chlorofluorocarbon (CFC) as the fluid within the housing. These liquids are generally undesirable organic solvents that are toxic and ozone depleting chemicals. Fluid-filled spherical shell reflective targets are therefore disadvantageous because they have potential damage to the environment due to the risk of fluid leaking into and contaminating the surrounding environment, resulting in limited use of the material. In addition, the reflective target of the fluid-filled spherical shell is relatively difficult and expensive to manufacture.

Another known acoustic reflector is a three-wing reflector, which typically includes three orthogonal reflection planes that intersect at a common origin. However, the reflectors may require coatings to make them acoustically reflective at the relevant frequencies and for use in marine environments, and despite having a high target strength, the reflective properties of the coating material tend to vary with pressure due to depth under water. In addition, the three-wing reflectors are disadvantageous in that their reflectivity depends on and is limited to their orientation, wherein the target intensity can vary by more than 6dB at different angles.

There is also a need for acoustic reflector tags suitable for attaching to, locating, tracking and monitoring marine milk-catching animals (such as seals, dolphins and whales) for research purposes, but such tags need to be lightweight and small in size so as not to restrain the animals in any way. The above-mentioned known reflectors are not suitable for this application. As mentioned above, liquid-filled spherical reflectors rely on toxic materials and are therefore considered potentially hazardous to the animals to which they are attached and the surrounding environment in which they live. The three-wing reflector is not omnidirectional but depends on and is limited to its useless orientation.

Applicant's british patent No.2,347,016 discloses and claims an acoustic reflector comprising a housing, the housing having a wall arranged to surround the core, the housing being capable of transmitting acoustic waves incident on the housing into the core, to focus and reflect acoustic waves from an area of the housing opposite the incident area, to provide a reflected acoustic signal output from the reflector, the acoustic reflector is characterized in that the core is in the form of a sphere or a right cylinder and is formed by one or more concentric layers of solid material having a wave speed of from 840 to 1500m/s, and the housing is dimensioned relative to the core such that a portion of the acoustic waves incident on the housing are coupled into the housing wall and guided therein about the circumference of the housing, and then re-radiated to combine constructively with the reflected acoustic signal output to provide an enhanced reflected acoustic signal output.

The reflector is durable, non-toxic, small in size and relatively easy and inexpensive to manufacture.

Note that the reflector may be in the shape of a sphere or a cylinder with a circular cross-section orthogonal to the generator. In the case of a cylinder, the reflector is in the form of a long continuous system (i.e. a rope) with a high sonar return from specular flicker from those parts of the rope that are arranged at right angles to the direction of travel of the acoustic signal.

The core may be formed from a single solid material. Alternatively, the core may comprise more than two layers of different materials, wherein for a particular selected frequency of the acoustic wave, the layers will provide more efficient focusing of the incoming wave and/or lower attenuation within the material, resulting in a stronger output signal overall. Suitable core materials have been described, noting that they should not exhibit high acoustic energy absorption in the operating region.

The housing may be formed of a rigid material such as a Glass Reinforced Plastic (GRP) material, in particular a glass filled nylon such as 50% glass filled nylon 66, or 40% glass filled semi-aromatic polyamide, or steel, and may be sized such that its thickness is approximately one tenth of the radius of the core. However, one skilled in the art will readily appreciate the deviation of these parameters with respect to the appropriate relationship between the properties of the materials used for the core and shell.

The principle of the combination of waves emitted through the reflector's shell and internally focused waves can be used within the design of the present device to provide highly identifiable characteristics to the enhanced reflected acoustic signal output from the device. For example, the signal output may be arranged to occupy a characteristic time stamp (timesignature) or spectral content.

By appropriately modifying the sonar (which is used to detect the acoustic signal output, thereby identifying a characteristic feature in the output), it is therefore easier to distinguish between signals and background clutter from the reflectors of the present invention, and returns from other (false) targets located in the field of view of the sonar detector employed.

It is also noted that by appropriately manipulating the phasing between the two returns (i.e. the geometrically focused return from the core and the elastic wave return from the outer shell), the device can be arranged to exhibit a unique resonant frequency that will "render" the returned echo. By this means, returns from a particular reflector can be distinguished from other (false) targets in a very cluttered environment.

Disclosure of Invention

The applicant has found that by appropriate selection of dimensions and materials, an acoustic reflector having a structure substantially as hereinbefore described can be made to exhibit the characteristic of having two or more separate transmission windows on separate regions of the housing to form two or more separate focused acoustic paths through the core of the reflector. The device provides enhanced reflected acoustic signal output by interference between different acoustic pathways formed by separate transmissive windows in the housing.

There is thus provided an acoustic reflector comprising a shell having a wall arranged to surround a core, the shell being capable of transmitting acoustic waves incident on the shell into the core to focus and reflect acoustic waves from an area of the shell opposite the incident area to provide a reflected acoustic signal output from the reflector, the core being formed from one or more concentric layers of solid material having a wave speed of from 840 to 1500m/s, the acoustic reflector being characterised in that the shell is dimensioned relative to the core such that incident acoustic waves are transmitted through the shell into the core along two or more different paths and the associated reflected signal outputs combine constructively to provide an enhanced reflected acoustic signal output at one or more predetermined frequencies.

The reflector is preferably in the shape of a sphere or cylinder with a circular cross-section orthogonal to the generator. In the case of a cylinder, the reflector is in the form of a long continuous system, i.e. a rope, with high sonar returns from specular glints from those parts of the rope that are arranged at right angles to the direction of travel of the acoustic signal. Alternatively, it has been found that reflectors of the above type may be more effective if they are ovoid (football) shaped (assuming a circular cross-section).

The reflector of the present invention can be tuned to a specified frequency by appropriate selection of core diameter, shell thickness, and corresponding material properties of the various components. In particular, it is important that the acoustic wave velocity of the inner core material is such that the two focused return signals have different acoustic path lengths so that structural interference can be formed between the signals.

Preferably, the core is formed from a single solid material having a wave speed between 840m/s and 1300 m/s. Alternatively, the core may comprise more than two layers of different materials, wherein for a particular selected acoustic frequency these will provide more efficient focusing of the incoming wave and/or lower attenuation within the material, resulting in a stronger output signal overall. However, it is naturally anticipated that the manufacturing complexity and cost will be greater in the case of a laminated core. When the core is formed of two or more layers of materials, the wave speed of either or both of the materials reaches 1500 m/s.

In order to be suitable for use in the reflector device of the present invention, the core material must be such that it exhibits a wave velocity within the required range without high absorption of acoustic energy. The core may be formed of an elastomeric material such as silicon, in particular RTV12 or RTV655 silicon rubber from Bayer or Alsil 14401 peroxide cured silicon rubber.

The housing may be formed of a rigid material, for example a Glass Reinforced Plastic (GRP) material, in particular glass filled nylon, such as 50% glass filled nylon 66 or 40% glass filled semi-aromatic polyamide, or steel, and may be dimensioned such that its thickness is approximately one tenth of the radius of the core.

To further influence the spectral response of the reflector, the internally focused wave may, if desired, be combined with a (elastic) wave transmitted through the housing of the reflector as previously described in applicant's british patent No.2,437,016, to provide one or more highly identifiable characteristics for the enhanced reflected acoustic signal output from the device.

There may also be advantages arising from the following features: the signal output of the reflector according to the invention may comprise a characteristic time stamp and can therefore be uniquely identified. In general, targets in the form of spheres can often be easily distinguished from a large number of false targets because they produce a very well-recognized "tail" to the return signal (echo). The echogenic structure is formed by a plurality of acoustic pathways within the reflector device and has a characteristic exact periodic structure that is not replicated by most underwater targets.

Due to the ability to produce a particular spectrum of echo returns, the spectral response of the reflector of the present invention (using optical analysis) is colored, rather than slightly monochromatic, as is the case with most underwater targets at frequencies typically used by sonar systems. Thus, it is very easy to distinguish between the signal returned from the reflector of the present invention and background noise and returns from other (false) targets located in the field of view of the sonar detector being used.

Additionally, however, many very useful applications of the apparatus of the present invention become apparent due to the ability to tune individual reflectors to produce different spectral outputs. For example, by using a sonar system operating in a dual frequency mode and tuned to two different reflector frequencies, the respective reflectors can be used as "traffic lights" or to define an exclusion zone for autonomous or semi-autonomous systems, or to provide a marine route for the vehicle in the form of a path between two rows of differently tuned reflectors.

It should be noted that because the echo return from the reflector according to the invention is completely independent of the geometry with respect to the interrogating sonar, the device can be configured to be location only important, and not important to how the device is placed on the seabed. Placing the reflector under water is therefore simpler, more efficient and less expensive to implement than with other more directional devices.

As an alternative to using a sonar operating in a dual frequency mode, a sonar operating in a broad band mode and using different frequency spectra to correlate two different colors to respective reflectors may be used. Although it is recognised that this may require appropriate modification of a conventional sonar system to provide sufficient bandwidth for illumination and appropriate signal processing capability to be able to detect different acoustic signal outputs (and hence provide enhanced recognition capabilities), it is nevertheless contemplated that only the latter may actually be required (i.e. development of some processing software).

Another possible application of the reflector of the present invention is the ability to be positioned relative to the known position of one or more reflectors. This is particularly useful for Autonomous Submersibles (AUVs) that rely on Inertial Navigation Systems (INS) for positioning. It is known that the INS of the machine needs to be recalibrated as the ship goes down in depth, and this can be achieved by interrogation of reflectors with known spectral characteristics and known positions. To help identify the particular reflector used to provide the reference location, a set of reflectors in a particular pattern may conveniently be provided, and this may be in the form of a pre-fabricated bond, for example on a board or pad. The same type of device may also be used to position related objects on the seabed, such as wellheads or pipe valves with different numbers and/or arrangements of reflectors that represent the specific purpose being marked.

Additionally, it should be noted that the sonar source may be mounted on any conventional carrier, such as a submarine or other man-driven submarine, a sonar permanently installed underwater, a pick-up sonar installed on a boat, airplane or helicopter, or an AUV.

In the present invention, an identification and recovery system for a seafloor object comprises a passive sonar reflector attached to the object, a sonar transmitter, and means for receiving sonar signals reflected from the passive sonar reflector. The receiving device may be co-located with the transmitter or located in some other location. Triangulation systems are also possible in which the receivers are located at three different positions and the specific position of the object is identified by conventional triangulation means.

Many novel applications of these systems are possible. These applications include:

alone or in combination with other similar sonar reflector/active positioning devices to mark a particular geographical location of an object under the sea or applied to an object ready for subsequent diving to aid in positioning (e.g., red and green versus red and blue), i.e., pipelines, cables, phone lines, fixed equipment on the sea floor;

for undersea devices or for devices ready for subsequent diving that will mark the current position of the device within or at the bottom of the water column or on the seabed, i.e. mark cables or other devices that move freely or to and fro within a certain range, such as certain cables or other movable resources that move with tides and/or flow;

marking the submerged parts of oil or gas platforms or the rest of these platforms, which may include the use of differently tuned reflectors as a means of identifying ownership, function or type, etc. of a particular class of submerged resources;

marking a location with seafloor/nautical significance, but where it is not necessary to attach sonar reflectors to specific equipment, e.g. for ocean routes, as an in-port location aid, for debris or other nautical threats, such as coral, underwater rock, etc.;

marking or indicating an economically or commercially relevant area, for example, a national offshore boundary for explaining mineral mining rights;

identifying the location and recovery of high value containers lost from the ship to the ship, or lost in the event of an aircraft crash, or aircraft black boxes;

geography monitoring, such as identifying and monitoring movement of rifles in the ocean.

Marking dangerous objects on the seabed for later processing such as debris and deposits.

A further possible application is to provide a means by which the position of a diver can be tracked from a sea-surface vessel, thus providing assistance to the diver when required. Although diver tracking systems currently exist for this purpose, these systems are typically based on powered active transducers. Such transducers are expensive and bulky compared to the passive acoustic reflectors of the present invention and require periodic recalibration and maintenance to keep the device operating reliably and correctly, whereas passive reflectors do not require recalibration or maintenance. Furthermore, by the ability to tune individual reflectors, more than one diver operates from the surface vessel, and each diver can be individually "tracked". The reflector can be tuned to correspond to standard depth or widely applicable and less expensive fish-finding sonar.

It should be noted that the size of the acoustic reflector of the present invention may be varied as desired. Larger devices will give a stronger return signal, but smaller reflectors (e.g., about 50 to 100mm in diameter) are preferred, for example, for attachment to divers or marine animals.

Drawings

The invention will now be described by way of example and with reference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a cross-section through an acoustic reflector according to the present invention showing some of the acoustic path through the reflector core;

FIG. 2 is a graph showing frequency versus target intensity for a particular combination of materials and dimensions for a shell and core of an acoustic reflector according to the present invention;

FIG. 3 is a graph of target intensity versus frequency for two different reflectors, illustrating the effect of different thicknesses of the housing wall on the frequency response;

FIG. 4 is a trace obtained using a commercially available fish finder apparatus, showing a number of reflectors lowered to a seabed location in accordance with the present invention;

fig. 5 is a photograph of the output from a multi-beam sonar scanning an area of the sea bed with two reflectors located between the water surface and the sea bed in accordance with the present invention; and

fig. 6 is a photograph of the output from a multi-beam sonar scanning an area of the sea floor with a set of five reflectors located near the sea floor in accordance with the present invention.

Detailed Description

Referring to fig. 1, an acoustic reflector 10 includes a spherical shell 12 having a wall 14. The wall 14 surrounds the core 16. The housing 12 is formed of a rigid material, such as a Glass Reinforced Plastic (GRP) material or steel. The core 16 is formed of a solid material such as an elastomer.

Sound waves 18 emitted from a sound source (not shown) are incident on the housing 12 as shown. The properties of the shell are chosen in the manner described above such that it presents two regions arranged around the weft of the shell (the two regions being treated as transmission windows), i.e. such that the incident acoustic waves are effectively transmitted through the shell wall 14 in these regions and into the core 16. Thus, as the incident sound waves travel and refract through the core 16, they follow two paths 19, 19' and are thereby focused on a region 20 of the housing on the side opposite to that on which the sound waves 18 are incident. The waves then reflect back along the same respective paths and combine together to provide an enhanced reflected acoustic signal output 22 of the reflector.

As shown in applicant's british patent No.2,437,016, for regions of the housing where the incident angle of the incident sound wave is small, a portion 18 of the incident wave is coupled into the wall 14 and generates an elastic wave 26 which is guided within the wall 14 around the circumference of the housing 12. Where the materials forming the housing 12 and core 16 and the relative dimensions of the housing and core are predetermined such that the propagation time of the housing wave 26 is the same as the propagation time of the internally geometrically focused return wave 19, 19', the elastic wave traveling through the housing wall and the reflected acoustic signal output are in phase with each other and therefore combine constructively at the target frequency to provide a further enhanced reflected acoustic signal output (i.e. a strong target response).

Fig. 2 shows data obtained by digital simulation, including a graph plotting target intensity (TS) against frequency (F) of an incident sound wave for a spherical sound reflector according to the present invention. In this case, the reflector is considered to include: a silicone rubber core having a density of 1.0g/cm³And the acoustic velocity is 1040 m/s; and a housing having a longitudinal wave velocity of 2877m/s, a shear wave velocity of 1610m/s and a shear wave velocity of 1.38g/cm adapted to the glass-reinforced polyamide material³The density of (c). The outer diameter of the reflector was set to 210mm, and the ratio of its inner diameter to its outer diameter was 0.942: 1.

As can be seen in the graph, the reflector in this case shows a high level of return, i.e. a higher target intensity, at many frequencies (approximately between 20kHz and 120kHz, in particular in the region of 25, 40, 80 and 110 kHz).

The data in fig. 3 was generated on the same basis as fig. 2, but showing spectral responses for two different reflectors having the same core and shell characteristics as the reflector of fig. 2, and an outer diameter of 210mm, but with different values for the ratio of inner to outer diameter (0.942 (thick line) and 0.838 (thin line) corresponding to shell thicknesses of 12mm and 34mm, respectively). As can be seen from fig. 3, the reflector according to the invention can be formed such that a single parameter variation of the shell thickness results in a reflector with a very distinct and different spectral response. Those skilled in the art will readily appreciate that other variations may be obtained by varying the material properties of the inner core and/or outer shell of the reflector.

The acoustic reflector used in obtaining the results shown in fig. 4 to 6 comprised a RTV12 silicone rubber core with an acoustic wave velocity of 1040m/s and a glass reinforced polyamide shell.

Figure 4 is a trace of an offshore test obtained in 30m water with a good seabed using a plurality of reflectors according to the present invention and a commercially available fish finder apparatus of 50 kHz. The traces are time and depth traces and the positions of the 5 reflectors are clearly shown as they are lowered to the seabed.

Fig. 5 is a photograph of the output from the multi-beam RESON 8111 Seabat sonar system. The sonar is held above the bow of the vessel, with the sonar head held 2m below the water surface, and then the vessel travels through the seabed area at a depth of 150m, positioning two reflectors according to the invention at a depth of between 70m and 80m above the area. The reflector shows a high response, can be easily selected against background noise, and is located above the sea-bed response. From this trace a map of the seabed can be formed to show the layout of the seabed and the location of the reflector.

Fig. 6 is an output photograph from a multi-beam depth sonar system scanning a region of the sea floor with a set of five reflectors located about 1m above the sea floor in accordance with the present invention. The area to the right of the reflector shows an area of rocky layers, the very good seabed elsewhere.

From experience with offshore testing, such as those described herein, it was determined that the acoustic reflectors described herein (operating at a maximum response frequency of 120 kHz) can be detected outside of at least the 800m range using commercially available sonar systems. The reflector according to the invention can thus provide a very efficient and low cost means for marking the position of objects on or near the seabed.

It has been found that the manufacture of the acoustic reflector of the present invention is facilitated by forming the two reflectors in two halves, which are then adhered together. The two halves are the same for spherical and ovoid reflectors. The production is generally carried out as follows. The half shells were first manufactured by injection moulding using Zytel material (Zytel 151LNC010), a polyamide suitable for moulding, supplied by DuPont. The molded shell was left for 24 hours and then internally degreased. The interior of each half shell is then treated with a primer to promote good adhesion to the core material (typically RTV silicone rubber) that is poured to fill the half shell. Suitable primers for use with these silicone rubber materials include products SS4004P, SS4044P, SS4120 or SS4155, available from GEBayer. For RTV12 rubber, the recommended base is SS4004P, and alternative bases are SS4044P or SS 4155.

Each filled half shell was then left at room temperature for a period of 2 to 14 days to cure the silicon core material into a solid. Conveniently, a catalyst is used to aid the curing process and efforts are made to ensure that by-products generated during the curing process are minimised; likewise, longer cure times also contribute to this. Suitable catalysts for this treatment include the product TRV12C 01P supplied by GB Bayer and the product TSE3663B supplied by Momentive Performance MaterialS S GmbH of Levokusen.

At this stage, any slight shrinkage due to curing of the silicone rubber material may be allowed by topping up with a further different core material and allowing the material to cure. Once the filled half shells are prepared exactly as described, an adhesive (such as Loctite 3425) is applied to the mating faces of the half shells and the two half shells are brought into contact and clamped together and then left at room temperature for 14 days to fully cure the adhesive.

After the curing period, each reflector is scanned (e.g., with a high resolution X-ray scanner) to inspect for voids or cracks in the reflector. Assuming no voids or cracks are detected, the scanned reflector units are calibrated in water all over a frequency range from 50kHz to 900 kHz. This calibration is performed by successively interrogating each reflector element with a plurality of pulses from the sonar within the target frequency band. The reflected response is measured and plotted against frequency. These measurements are repeated for each rotational position of the unit relative to the sonar position, each position being 10 ° apart, i.e. for a total of 36 measurements. The reflector was then rotated 90 ° in the other plane and the 36 measurements repeated. A certificate of certification listing the performance characteristics of the reflectors may thus be prepared for each reflector unit.

Claims (11)

1. An acoustic reflector comprising a core and a shell surrounding the core, the shell having a wall arranged to surround the core and two or more separate transmission windows on separate regions of the shell, the shell transmitting acoustic waves incident on the shell wall into the core to focus and reflect the acoustic waves from a region of the shell wall opposite the region of incidence of the acoustic waves to provide a reflected acoustic signal output from the reflector, the core having a circular cross-section and being formed from one or more concentric layers of solid material having a wave speed of 840 to 1500m/s, the acoustic reflector being characterised in that incident acoustic waves are transmitted through the shell wall into the core via the two or more separate windows on the shell along two or more separate paths and the associated reflected acoustic signal outputs combine constructively, to provide enhanced acoustic signal output at more than one predetermined frequency.

2. An acoustic reflector, according to claim 1, wherein the core is in the form of a sphere or right cylinder.

3. An acoustic reflector according to claim 1, wherein the core is formed from a single solid material having a wave speed between 850m/s and 1300 m/s.

4. An acoustic reflector according to claim 1, wherein the core is formed from an elastomeric material.

5. An acoustic reflector, as set forth in claim 4, wherein the elastomeric material is silicone rubber.

6. An acoustic reflector, as set forth in claim 5, wherein the elastomeric material is RTV12 silicone rubber or RTV655 silicone rubber.

7. An acoustic reflector, as set forth in claim 1, wherein the housing is formed of a rigid material.

8. An acoustic reflector, as claimed in claim 7, wherein the rigid material is a steel or Glass Reinforced Plastic (GRP) material.

9. An acoustic reflector, as claimed in claim 7, wherein the rigid material is glass-filled polyamide or glass-filled nylon.

10. An acoustic reflector, as set forth in claim 1, wherein the signal output is further characterized by a specific time stamp.

11. An acoustic reflector according to claim 1, wherein the shell is dimensioned relative to the core such that a portion of the acoustic waves incident on the shell are coupled into the shell wall and pass around the core in the shell and are then re-radiated to combine constructively with the internally reflected acoustic signal output.